1. Field of the Invention
The present invention relates to the field of analyzing and predicting the behavior of mechanical components.
2. Brief Description of the Related Art
The components in gas turbines (moving blades, guide blades, liners, etc.) are as a rule so highly stressed that they have only a finite service life. It is necessary to predict this service life for a reliable and economical design of gas turbines.
The loading of the components is composed of forces, high thermal loads, oxidation and corrosion. The mechanical and thermal loading leads in many cases to fatigue of the components after just a few thousand load cycles. This low cycle fatigue is reproduced in the isothermal case by LCF tests (LCF=Low Cycle Fatigue) and in the anisothermal case by TMF tests (TMF=Thermal Mechanical Fatigue).
The stresses caused by the loading are determined in the design phase of the gas turbine. The complexity of the geometry and/or of the loading requires the use of the finite element (FE) method for determining the stresses. However, since inelastic calculations, which are often necessary, are as a rule not possible for reasons of cost and time, the service life prediction is based almost exclusively on linear-elastic stresses. Usually only isothermal data (strain-controlled LCF tests) are available, for which reason anisothermal cycles have to be evaluated with LCF data.
In this case, the amplitude of the total equivalent strain εv,ep is used as a measure of the damage (damage law). If the requisite number of cycles Nreq in the component is to be achieved, the amplitude of the total equivalent strain εv,ep at each location of the component must satisfy the equationεv,ep≦εαM(Tdam, Nreq)  (a)where εMα is the admissible total strain amplitude which is determined from isothermal LCF tests. It is to be determined for various temperatures and cycle numbers. The temperature Tdam which gives rise to this damage be suitably selected for a cycle with varying temperature.
If the decisive loading at high temperatures acts for several minutes, additional damage can be expected. In order to be able to detect the reduced service life on account of the accumulation of damage by creep fatigue and cyclical fatigue, LCF data are obtained from tests with retention time.
The measure of damage εv,ep corresponds to the strain amplitude of a balanced cycle. This cycle is determined from the cycle analyzed in a linear-elastic manner via a modified Neuber rule:σ*dev·ε*(σ*dev)=σepdev·εep(σdev)  (b)where
σ*dev is the vector of the deviator of the linear-elastic stress amplitude
ε(σ*dev) is the vector of the linear-elastic strain amplitude
σdevep is the vector of the deviator of the total elastic-plastic strain amplitude, and
εep(σdev) is the vector of the elastic-plastic strain amplitude
The degree of damage εv,ep is determined via an equivalent hypothesis from the vector of the total elastic-plastic strain amplitude εep(σdev).
The cyclic σ-ε curve necessary for determining the total elastic-plastic strain amplitude εep(σdev) is represented analytically by a modified Ramberg-Osgood equation.
The inelastic effects occurring in gas turbine components (blades, combustion chambers) can then be detected approximately by means of the Neuber rule. These effects have to be taken into account in the service life prediction of the designs. However, only the Neuber rule (b) for materials with isotropic mechanical behavior has been known hitherto.
Since, in gas turbine construction, (anisotropic) single-crystal materials are increasingly used in the components, specifically the turbine blades, on account of their special properties, it would be desirable for the design of the components—in particular with regard to the determination of the service life under cyclic loading—to have a calculation method analogous to the case of isotropic materials.